Various portable electronic apparatus, such as cellphones, digital cameras, video cameras, and notebook computers and the like, use as the power source a secondary battery that can be used repeatedly through charging. Because of the requirements of small size and long hours of use, lithium ion batteries, having a high energy density, are widely used as such secondary battery. However, because lithium ion batteries have a risk of decreased battery performance or failure when overcharged, an overcharge needs to be prevented when charging.
Thus, in Japanese Unexamined Patent Application, First Publication 2001-313202, a protection device is disclosed to protect a lithium ion secondary battery when an overcharge occurs. That is, a PTC device and a fuse (low melting point metal) are provided in the charge circuit in this protection device so that when the lithium ion secondary battery is in an overcharged state, the resistance value of the PTC device increases to suppress the charge current to the lithium ion secondary battery. Further, in this protection device, when the temperature of the PTC device becomes excessive owing to heat being stored through repeated actuation of the PTC device, the above fuse is fused to cut off charge current to the lithium ion battery.
For example, a protection element is disclosed in Japanese Unexamined Patent Application, First Publication 2002-150918, wherein a PTC device is connected sandwiched between a pair of external connection terminals, with either of the external connection terminals having a fuse function. Further, in this protection element, the above fuse function is set to operate when the temperature condition reaches a temperature higher than the actuating temperature of the PTC device.
In the above protection device and protection element, after the PTC device is actuated by heat generated by a battery through being overcharged, the overcharged battery must generate more heat in order for the low melting point metal to operate. In order to remedy this, a means may be considered of making the low melting point metal operate immediately by adopting a low melting point metal with a lower melting point. However, this would cause the low melting point metal to fuse simply because the ambient temperature rose through natural use and therefore cannot be offered for practical use.
This invention was made in view of the above circumstances and has the purpose of providing a means of preventing a secondary battery from failure or decreased performance caused by overvoltage on the secondary battery during charging.